A major problem in the long distance transmission of optical signals over an optical fibre path is that of dispersion of the transmitted signal. Dispersion arises from differences in the velocity of the various frequency and/or modal components of the signal within the transmission medium. This effect causes broadening of the component pulses of a signal and thus limits the distance over which signals may be transmitted before regeneration becomes necessary. The problem can be reduced by an appropriate choice of transmission frequency or wavelength. It has been found that, for a silica optical fibre there is a wavelength at which the signal dispersion has a minimum (non-zero) value. Transmission of signals at this frequency results in some improvement but is by no means sufficient to allow transmission over very long distances. Conventionally this problem is addressed by providing repeaters at regular intervals along the fibre optic path. A typical repeater provides regeneration and retiming of the optical signal and may also provide a supervisory function e.g. for error checking. Thus, the conventional repeater is a complex and somewhat costly device. Further, because such a repeater is designed to be compatible with the system signalling format and bit rate, a system provided with such repeaters cannot subsequently be upgraded, e.g. to a higher bit rate, without considerable inconvenience and expense. Indeed, for a submarine system where recovery and replacement of repeaters is impractical, subsequent system upgrading may be impossible.
In an attempt to overcome this problem it has been proposed that optical signals be transmitted in a soliton format. A soliton is a solitary wave or pulse that propagates over very long distances with substantially no deterioration. The generation of optical solitons has been described by L. Mollenauer & K. Smith in "Demonstration of Soliton Transmission over more than 4,000 Km in fibre with loss periodically compensated by Raman gain" Optical Letters, 13 (1988) page 675.
A significant problem in the transmission of an optical signal in a soliton format is that of amplitude control. Soliton transmission via an optical fibre path requires a precise signal amplitude which depends on the fibre dispersion characteristics and on its effective cross-section. Departure from this amplitude results in a reduction of soliton purity with a consequent loss of dispersion-free transmission.
An object of the invention is to minimise or to overcome this disadvantage.
A further object of the invention is to provide an improved fibre optic transmission system.
One solution to the problem of long distance soliton transmission is proposed in our co-pending application No. 89 25818 0 (K. C. Byron 40) This describes a method of eliminating soliton-type interaction in an optical fibre communications system comprising the step of causing the optical fibre to appear, to a signal to be transmitted thereby, to be a passive optical pipe.
The present invention provides an alternative approach to the problem of soliton transmission.